Tertiary amine boranes are utilized in many fields, which include the preparation of epoxy resins by acting as a curing agent or accelerators; in organic synthesis as reducing agent; as a reagent for the hydroboration of alkenes and in electroless plating applications. Essentially five methods have been developed for the synthesis of tertiary amine-boranes and these methods are described as follows:
1. The direct reaction of an amine and diborane;
2. Reacting tetrahydroborates with ammonium salts;
3. Transamination of aminoboranes;
4. The reaction of a metal tetrahydroborate with iodine in the presence of an amine; and
5. The reaction of a metal tetrahydroborate with carbon dioxide or carboxylic or mineral acid in the presence of an amine and organic solvent.
Many of the above methods were difficult to practice in the production of tertiary amine-boranes because of the required synthesis of various reactive materials, the handling of expensive or difficult to handle reagents and difficulty in product recovery.
Of the above synthesis techniques, the one which appears to be most widely utilized is synthesis route 5 which involves the reaction of a metal borohydride with a tertiary amine. The following patents are representative:
U.S. Pat. No. 3,013,016 discloses a process embraced within synthesis route 5 for producing trialkylamine boranes. A trialkylamine, e.g. trimethylamine or triethylamine, is contacted with potassium borohydride in the presence of an aqueous phase containing an inert organic solvent which is immiscible or only partially miscible with water. Carbon dioxide, or an acid source such as a carboxylic acid or mineral acid is added to the reaction medium. Reaction occurs at about 20.degree.-40.degree. C. and atmospheric pressure. The product is recovered by separating the organic solution containing the trialkylamine borane for the aqueous phase and then separating the organic solvent from the trialkylamine borane by distillation.
Czechoslovakian patent 242,064 discloses a process for producing tertiary amine-boranes via synthesis route 5 wherein an anhydrous suspension of a metal tetrahydroborate and tertiary amine in an organic solvent is contacted with carbon dioxide gas at temperatures ranging from about 0.degree. to 50.degree. C. for a period of 1-5 hours. The reaction product is washed with water, dried and the solvent evaporated.
British patent 909,390 discloses a process for producing amine boranes by reacting an amine salt with aqueous metal borohydride in organic solvent at room temperature under neutral or alkaline conditions. Product recovery is accomplished by separating the amine borane as it is formed by separating organic solvent containing the amine borane from the aqueous layer and then distilling the solvent from the amine borane.
Taylor, et al. in J. Am. Chem. Soc., vol. 77, 1955, pp. 1506-7 disclose the production of pyridine-borane complexes by reacting pyridine hydrochloride with sodium borohydride in the presence of pyridine solvent. Byproduct sodium chloride precipitates and unreacted pyridine hydrochloride is precipitated by the addition of ether. Unreacted pyridine is separated from the pyridine-borane reaction product by distillation.